A spark plug for an internal combustion engine is known, which includes a metal shell formed with a tool engagement portion and a mounting thread portion, a ceramic insulator (insulator) inserted in a through hole of the metal shell in an axial direction, a center electrode fixed in the ceramic insulator and a ground electrode fixed to a front end portion of the metal shell so that the spark plug can generate a spark discharge between a front end portion of the center electrode and the ground electrode.